Abstract Toxicity assays based on human organs-on-a-chip platforms have become increasingly important for drug discovery and development, since they allow testing cytotoxic effects of pharmaceutical compounds on the physiologically relevant human tissue models before moving forward to expensive animal testing or clinical trials. Multiple physiological and biochemical parameters of the organ-on-a-chip models must be continually monitored in order to assess the responses of these models to drug treatments. Although fluorescence detection has been widely adopted for bioassays, it requires the addition of fluorophores to the samples, which may disturb cellular activities and more seriously, it is practically impossible for real-time fluorescence labeling of the biomarkers that are constantly secreted by the organ models during drug toxicity testing. Thus, fluorescence detection is not a viable option here - direct detection, or ?label-free? detection, is required for monitoring the dynamic process of drug interactions with organoids to obtain detailed information on transient as well as delayed or cumulative drug effects. The overarching goal of the proposed research is thus to address these challenging issues of drug toxicity assays by using a human organ-on-a- chip model monitored with an automated, label-free, optical biosensor system that allows for real-time, long- term, sensitive, and kinetic analyses of human cardiac tissue models in response to various drugs in their microenvironments. To accomplish this goal, we propose a unique approach that is based on our patented label-free biosensor in conjunction with advanced organ-on-a-chip technologies. The open-microcavity configuration of our biosensor enables synergistic integration of the sensor chip with a heart-on-a-chip model through an automated microfluidic platform, which has the built-in capability to regenerate the sensor surface for continual kinetic studies over extended periods of time. The heart-on-a-chip model will be developed using an innovative 3D bioprinting approach that produces functional biomimetic cardiac organoids using cardiomyocytes derived from human induced pluripotent stem cells (iPSCs). A microfluidic perfusion bioreactor with the built-in capacity for simultaneous electrical and mechanical stimulations will be constructed to maintain long-term functionality of the organoids. On the other hand, the long-term stability of the proposed biosensor system will be significantly enhanced using negative thermal expansion materials for fabrication of the sensor chip. The cardiotoxicity of a panel of drugs will be evaluated in situ via quantification of the biomarkers secreted by the human cardiac model. The technology developed from this project will be highly transformative, which may be applied for other organs and lead to future personalized screening of drug toxicities, efficacy, and pharmacokinetics for precision medicine.